A differential interference microscope is used to observe microscopic detail in specimens. The image formed in such a microscope represents the gradient of optical paths for both low and high spatial frequencies and may be a monochromatic shadow-cast image or an image with various background colors. With reference to FIG. 1, a conventional (i.e., prior art) reflected light differential interference microscope 10 comprises, along a first optical axis A1, a light source 11, a collector lens 12 and a polarizer P. A beam splitter BS is located adjacent polarizer P at the intersection of optical axis A1 and a second optical axis A2 arranged perpendicular to optical axis A1. Adjacent beam-splitter BS along optical axis A2 is a birefringent optical member in the form of a Wollaston prism W having a ray-separating plane Q.sub.W which intersects optical axis A2 at an intersection point P.sub.W. Adjacent prism W is an objective lens 13 having a rear focal point F.sub.13 and an object plane OP within which is located a specimen (object) 14 to be observed. On the side of beam-splitter BS opposite Wollaston prism W and along axis A2 is an analyzer A and an image plane IP.
Microscope 10 operates as follows. Light (i.e., light beam) L.sub.1 from light source 11 is converged by collector lens 12 which directs the light through polarizer P thereby linearly polarizing the light. Polarized light L.sub.1 then impinges beam splitter BS, which reflects the light to Wollaston prism W. In prism W, polarized light L.sub.1 is separated by birefringent action into two linearly polarized components represented by rays R.sub.o and R.sub.e having mutually orthogonal oscillation directions. After passing through prism W, rays R.sub.o and R.sub.e travel divergently with a small separation angle .alpha. as if they were apparently separated at ray-separating plane Q.sub.W. Rays R.sub.o and R.sub.e then proceed toward objective lens 13, which also serves as a condenser lens. Diverging rays R.sub.o and R.sub.e are made parallel by the converging action of objective lens 13, with the rays being separated by a small shear amount S. Parallel rays R.sub.o and R.sub.e then illuminate specimen 14 at slightly separated positions.
With continuing reference to FIG. 1, rays R.sub.o and R.sub.e are reflected from specimen 14 at the slightly separated positions and are converged onto ray-separating plane Q.sub.W by the convergent action of objective lens 13. Rays R.sub.o and R.sub.e are then synthesized into one light beam L.sub.2 by the birefringent action of prism W. Synthesized light beam L.sub.2 then travels in reverse on the identical optical path, and passes through beam splitter BS to analyzer A. Only the components of light beam L.sub.2 that oscillate in the identical direction pass through analyzer A. These components then interfere, giving rise to interference fringes (not shown) corresponding to the phase difference imparted to rays R.sub.o and R.sub.e upon reflecting from specimen 14. The interference fringes are observed at image plane IP as a magnified image 15.
With reference now to FIG. 2, prior art Wollaston prism W of microscope 10 (FIG. 1) is constituted by cementing two wedge prisms Wa and Wb, both formed from birefringent optical material. Examples of such material is a crystal like quartz or calcite. Prism wedges Wa and Wb are combined such that their respective optic axes are mutually orthogonal. More particularly, prism W is formed so that optic axis c of wedge prism Wa (i.e., the entrance-side prism) is in the x-axis direction and optic axis d of wedge prism Wb (i.e., the exit-side prism) is in the y-axis direction. The z-axis is the travel direction of a perpendicularly impinging light ray R. The y-axis is the axis orthogonal to the z-axis in the paper plane and the x-axis is the direction orthogonal to and into the paper plane. The wedge angle .theta. is in the y-z plane, hereinafter called the "wedge plane." Wedge angle .theta. is defined by normal line n.sub.M of entrance surface M of prism W and the normal line n.sub.C of joining surface C of wedge prisms Wa and Wb.
Referring to FIGS. 1 and 2, light ray R impinges perpendicular to prism W at entrance surface M and is subject to the birefringent action of the birefringent material constituting prism W.
This action separates light ray R into aforementioned two linearly polarized light rays R.sub.o and R.sub.e having mutually orthogonal oscillation directions as indicated by the hash marks on ray R.sub.e and the dots on ray R.sub.o. Light rays R.sub.o and R.sub.e exit from prism W at exit surface EXW with separation angle .alpha.. However, the point of separation of rays R.sub.o and R.sub.e when viewed from the exit surface EXW side is apparently point P.sub.W in ray-separating plane Q.sub.W.
Accordingly, in microscope 10 (FIG. 1), ray-separating plane Q.sub.W may be arranged so that intersection point P.sub.W substantially coincides with rear focal point F.sub.13 of objective lens 13. This causes light rays R.sub.o and R.sub.e separated upon exiting from prism W, as previously mentioned, to be converted to parallel rays by objective lens 13. This allows the reflected light from specimen 14 to be once again correctly superimposed by prism W. As a result, a differential interference image of high contrast is obtained.
If objective lens 13 is constituted by a plurality of lens groups, the focal point F.sub.13 may be formed inside a lens group. In particular, in the objective lens of a microscope optical system, the rear focal point is often located inside the lens. In contrast, since apparent ray-separating plane Q.sub.W necessarily exists inside prism W, it is impossible to arrange the prism at focal point F.sub.13 inside a lens group. Consequently, if focal point F.sub.13 is to be formed inside a lens group, a Nomarski prism is used in place of Wollaston prism W as the birefringent optical member.
With reference now to FIG. 3, Nomarski prism N is formed so that optic axis e of entrance-side wedge prism Na is in the x-axis direction. In addition, optic axis g of exit-side wedge prism Nb is in the y-z plane in a direction inclined by a predetermined angle .epsilon. with respect to exit surface EXN. Such a formation allows for a ray-separating plane Q.sub.N to be formed outside prism N. Accordingly, if focal point F.sub.13 is formed inside a lens group, it is preferable to use prism N, which can be made to act in the same manner as prism W by arranging ray-separating plane Q.sub.N formed outside of prism N to intersect rear focal point F.sub.13 of objective lens 13 (see FIG. 1).
In prior art microscope 10 of FIG. 1, it is preferable that image 15 be observed based on various background colors by varying the interference color. To do so, a phase difference is intentionally imparted between rays R.sub.o and R.sub.e to continuously vary the interference color. This imparted phase difference is in addition to the phase difference imparted to rays R.sub.o and R.sub.e upon reflecting from specimen 14.
There are several known methods of continuously varying the background color in the interference image in differential interference microscopes such as microscope 10. A first method involves inserting into the optical path a prism comprising a birefringent material and a so-called compensator plate, or a phase shifting element that combines plane parallel plates to vary the phase difference.
A second method imparts a phase difference by moving the birefringent optical member parallel along the direction of the line of intersection of the plane orthogonal to optical axis A2 and the plane that includes normal line n.sub.M of entrance surface M and the normal line n.sub.C of joining surface C (i.e., the y-direction), and by varying the ratio of the optical path lengths wherein the polarized rays R.sub.o and R.sub.e pass through entrance-side wedge prism Na and the exit side wedge prism Nb, respectively.
The first method is problematic because it requires a complex microscope construction. This results in high manufacturing cost due to the addition of a separate compensator plate or phase shifting element. The second method has the advantage that the background color can be varied without the addition of new parts. Accordingly, this second method is often adopted in differential interference microscopes. However, this method has other problems. For example, the ray-separating plane Q.sub.W of Wollaston prism W or the ray-separating plane of Q.sub.N of Nomarski prism N when used as the birefringent optical member is generally inclined in the wedge-angle .theta. direction of the prism (namely, in the wedge plane) by an angle .beta..sub.W or .beta..sub.N, respectively, as illustrated in FIGS. 2 and 3. Further, inclination angle .beta. differs depending on the construction of the prism. Inclination angle .beta..sub.W of ray separating plane Q.sub.W is comparatively small (and may be .beta..sub.W =0), and so may be ignored. However, it is known that inclination angle .beta..sub.N of ray separating plane Q.sub.N is extremely large compared with that of prism W. Consequently, the size of inclination angle .beta..sub.N cannot generally be ignored.
As mentioned above, objective lens 13 generally comprises a plurality of lens groups. Accordingly, it is often the case that its rear focal point F.sub.13 exists inside a lens. Using the fact that ray-separating plane Q.sub.N is formed outside of prism N, it is often the case that prism N is generally used as the birefringent optical member on the objective lens 13 side. With reference now to FIGS. 4 and 5, prism N is moved parallel along the direction of the line of intersection of the wedge plane and the plane orthogonal to optical axis A2 (i.e., parallel to the y-axis).
FIG. 4 shows the standard state wherein a prism central axis J is arranged on optical axis A2. In this state, intersection point P.sub.N of ray-separating plane Q.sub.N exactly substantially coincides with rear focal point F.sub.13 of objective lens 13. On the other hand, FIG. 5 shows a state wherein prism N is moved in parallel by an amount .DELTA.y along the y-direction for the purpose of varying the interference color in image 15 (FIG. 1). In this state, intersection point P.sub.N and rear focal point F.sub.13 of objective lens 13 are displaced on optical axis A2 by an amount .DELTA.z. Such a displacement between intersection point P.sub.N and rear focal point F.sub.1 of objective lens 13 degrades and reduces the contrast of image 15.